Method and apparatus for testing transmission lines normally propagating optical signals

ABSTRACT

A portable apparatus for measuring optical powers of optical signals propagating concurrently in opposite directions in an optical transmission path between two elements, at least one of the elements being operative to transmit a first optical signal (S 1 ) only if it continues to receive a second optical signal (S 2 ) from the other of said elements, comprises first and second connector means for connecting the apparatus into the optical transmission path in series therewith, and propagating and measuring means connected between the first and second connector means for propagating at least the second optical signal (S 2 ) towards the one of the elements, and measuring the optical powers of the concurrently propagating optical signals (S 1 , S 2 ). The measurement results may be displayed by a suitable display unit. Where one element transmits signals at two different wavelengths, the apparatus may separate parts of the corresponding optical signal portion according to wavelength and process them separately.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.14/468,890 filed Aug. 26, 2014 as a Continuation of U.S. patentapplication Ser. No. 13/204,350 filed Aug. 5, 2011 as a Continuation ofU.S. patent application Ser. No. 11/713,735 filed Mar. 5, 2007 as aContinuation-in-Part of U.S. patent application Ser. No. 10/538,768filed Jun. 10, 2005 as a Continuation-in-Part of International patentapplication No. PCT/CA2004/001552 filed Aug. 23, 2004 which designatedthe United States of America, and claimed priority from U.S. Provisionalpatent application No. 60/511,105 filed Oct. 15, 2003. The entirecontents of each of these prior related applications are incorporatedherein by reference.

TECHNICAL FIELD

The invention relates to a method and apparatus for testing transmissionlines normally propagating optical signals and is especially, but notexclusively, applicable to a method and apparatus for measuring opticalpowers of optical signals in optical transmission lines of passiveoptical networks.

BACKGROUND ART

As the cost of optical fiber and associated components decreases, newtelecommunications network deployments increasingly use optical fiberfrom the edge of a core network to a location at or very close to theend user. Such so-called FTTX (Fiber-to-the-X; where X is the home, theoffice, the building, the premises, the curb, etc.) installations areusually based on a passive optical network (PON) architecture, where aterminal at the core-network edge (Optical Line Terminal—OLT) broadcastssignals downstream along a fiber-optic cable to a N-port splitter, andeach of the ports then terminates at an optical network terminal (ONT)located at a respective one of the end users' premises. Typically,downstream signals are at either of two wavelengths, vis. 1490 nm forthe downstream transmission of digital data and 1550 nm for thetransmission of cable television (CATV) signals, while each end user'soptical network terminal (ONT) transmits upstream data signals at awavelength of approximately 1310 nm. It should be noted that the CATVsignals are often transmitted in analog format.

An asynchronous transfer mode (ATM) or similar protocol is often used toencode the downstream and upstream data signals. The OLT includes in thedownstream 1490-nm signals synchronization signals which permit each ofthe ONTs to send its upstream (1310-nm) signals in its own unique timeslot so as to avoid interference with signals from other ONTs connectedon the PON. For this reason, as well as for reasons of eye safety, thereis no 1310-nm transmission from the ONTs when the fiber link isdisconnected, thereby preventing reception of the 1490-nmdownstream-data signal.

Field maintenance of such FTTX installations requires low-cost andeasy-to-use diagnostic test instruments to measure the signals. Anexample of such diagnostic test instruments is an optical power meterthat can independently measure the power at the distinct downstream andupstream signal wavelengths (e.g. 1310 nm, 1490 nm, 1550 nm). During arepair call, the results of such a measurement could indicate the sourceof possible trouble in the network or in the end-user's connection. Itis also known to use optical spectrum analyzers (OSA) to measure opticalpower at several wavelengths at the same time.

A disadvantage of each of these instruments is that it is a one-portdevice that only measures the power if the signals at the differentwavelengths are propagating in the same direction along the fiber. Inthe case of the OSA, a further disadvantage is that the instrument isgenerally much too costly and complicated for routine fieldapplications.

SUMMARY

The present invention seeks to eliminate, or at least mitigate, thedisadvantages of the prior art instruments, or at least provide analternative and, to this end, provides a portable instrument formeasuring parameters, e.g. optical power, of analog or digital opticalsignals that are propagating concurrently in opposite directions in anoptical transmission path between two elements, such as network elementsof a passive optical network, at least one of which elements will nottransmit its optical signals if it ceases to receive signals from theother of the two elements.

According to one aspect of this invention, there is provided portableapparatus for connecting into (tapping) an optical transmission linenormally carrying optical signals propagating concurrently in oppositedirections, said apparatus comprising first and second connector meansfor connecting the apparatus into the optical transmission path inseries therewith, and propagation means connected between the first andsecond connector means for propagating said at least one of said signalswhile extracting a portion of either said at least one of said signalsor another of said signals.

The propagation means may also comprise means for detecting andprocessing said portion to determine the optical power of said at leastone of said signals or another of said signals.

According to another aspect of the present invention, there is providedportable apparatus for measuring the optical power of optical signalsnormally propagating concurrently in opposite directions in an opticaltransmission path between two elements, at least one of the elementsbeing operative to transmit a first optical signal (S1) only if itcontinues to receive a second optical signal (S2) from the other of saidelements. The apparatus comprises first and second connector means forconnecting the apparatus into the optical transmission path in seriestherewith, and propagating and measuring means connected between thefirst and second connector means for propagating at least the secondoptical signal (S2) towards the one of the elements, and measuring theoptical power of the concurrently propagating optical signals (S1, S2).

The measurement results may be displayed by a suitable display unit.

Where one element transmits signals at two different wavelengths, theapparatus may separate parts of the corresponding optical signal portionaccording to wavelength and process them separately.

According to another aspect of the present invention, there is provideda portable test instrument for measuring an optical power of each of afirst optical signal (S1) and a second optical signal propagatingconcurrently in opposite directions in an optical transmission path (16,16/1, . . . , 16/9) between two network elements (10, 14/1, . . . ,14/9), at least one (14/1, . . . , 14/9) of the network elements beingoperative to transmit the first optical signal (S1) only if it continuesto receive the second optical signal (S2) from the other (10) of saidnetwork elements, the portable test instrument comprising first andsecond connector means (22, 24) for temporarily connecting the portabletest instrument into the optical transmission path in series therewith,and propagating and measuring means (32, 38, 46) connected between thefirst and second connector means for propagating at least said secondoptical signal (S2) towards said at least one (14) of the networkelements, the propagating and measuring means (32, 38, 46) beingoperable to measure an optical power of each of the first optical signal(S1) and the second optical signal (S2). The propagating and measuringmeans comprises coupler means (32) having first and second ports (28,30)connected to the first and second connector means, respectively, so thata path therebetween within the coupler means completes the opticaltransmission path, a third port (36) for outputting a portion of eachoptical signal received via the second port (30) and a fourth port (34)for outputting a portion of each optical signal received via the firstport (28) and detection means (38, 42, 44) coupled to the third andfourth ports for converting the optical signal portions intocorresponding electrical signals.

Where said one of the network elements also normally receives via saidoptical transmission path a third optical signal (S3) at a differentwavelength from that of said second optical signal (S2), the propagatingand measuring means may further comprise means for measuring an opticalpower of the third optical signal (S3).

Where said one of the network elements also normally receives via theoptical transmission path a third optical signal (S3) at a differentwavelength to that of said second optical signal (S2), the propagatingand measuring means may further comprise a wavelength discriminator, forexample a wavelength division multiplexer, connected to the couplermeans for separating at least a portion (S2′, S3′) of the combinedsecond and third optical signals (S2, S3) according to wavelength toobtain corresponding separate parts (S2″, S3″) and supplying same tosaid detection means.

Alternatively, where said one of the network elements also normallyreceives via the optical transmission path a third optical signal (S3)at a different wavelength to that of said second optical signal (S2),the propagating and measuring means may further comprise a splitterconnected to the coupler means for splitting corresponding opticalsignal portions (S2′, S3′) into two parts (S2″, S3″), and filter meanscoupled to the splitter for separating the two parts according towavelength before supplying same to said detection means.

Where the optical signals are analog, the measuring means may bearranged to extract the time-averaged optical power of the signal.

Where the optical signals comprise bursts of digital data alternatingwith lulls, the measuring means may be arranged to extract the opticalpower averaged over the duration of the individual bursts. Moreparticularly, where the instrument is to be used for measuring power ofoptical signals comprised of “bursty” data streams (such as the ATM datasignals), the measuring means may be arranged to extract the power onlyfrom the data bursts and not from any intervening series of digitalzeros (i.e. lack of signal). Such bursty data streams are typical ofboth the upstream data sent by an optical network terminal (ONT) to aplurality of optical line terminals (OLTs) of a passive optical network(PON), and by the OLT to the plurality of ONTs.

The measuring means may comprise custom circuitry and/or asuitably-programmed microcomputer.

The test instrument may comprise display means for displaying measuredvalues of optical power.

According to yet another aspect of the invention, there is provided amethod of measuring an optical power of each of a first optical signal(S1) and a second optical signal propagating concurrently in oppositedirections in an optical transmission path (16, 16/1, . . . , 16/9)between two network elements (10, 14/1, . . . , 14/9), at least one(14/1, . . . , 14/9) of the network elements being operative to transmitthe first optical signal (S1) only if it continues to receive the secondoptical signal (S2). The method comprises the steps of: temporarilyconnecting first and second connector means (22, 24) of a portable testinstrument into the optical transmission path in series therewith, andusing the portable test instrument to propagate at least said secondoptical signal (S2) towards said at least one (14) of the networkelements, and measure an optical power of each of the first opticalsignal (S1) and the second optical signal (S2). The portable testinstrument comprises coupler means (32) having first and second ports(28,30) connected to the first and second connector means, respectively,so that a path therebetween within the coupler means completes theoptical transmission path, a third port (36) for outputting a portion ofeach optical signal received via the second port (30) and a fourth port(34) for outputting a portion of each optical signal received via thefirst port (28) and detection means (38, 42, 44) coupled to the thirdand fourth ports for converting the optical signal portions intocorresponding electrical signals.

Where said one of the network elements also receives via the opticaltransmission path a third optical signal (S3) at a wavelength differentfrom that of said second optical signal (S2), the propagating andmeasuring step may also measure an optical power of the third opticalsignal (S3).

Where said one of the network elements also receives via the opticaltransmission path a third optical signal (S3) co-propagating with thesaid second optical signal (S2) at a wavelength different from that ofthe said second optical signal (S2), the measuring step may furtherinclude the steps of splitting a portion of the co-propagating opticalsignals into two parts, each comprising portions of the second and thirdoptical signals (S2, S3), separating each of the two parts according towavelength, converting said parts into said second electrical signal anda third electrical signal, respectively, and also processing the thirdelectrical signal to obtain a measured value of an optical power of thethird optical signal (S3).

Alternatively, where said one of the network elements also receives viathe optical transmission path a third optical signal (S3) co-propagatingwith the said second optical signal (S2) at a wavelength different fromthat of the said second optical signal (S2), said propagating andmeasuring step may employ a wavelength discriminator, for example awavelength division multiplexer, connected to the coupler means forsplitting a portion of the co-propagating optical signals into two partseach corresponding to a respective one of the second and third opticalsignals, converting the parts to said second electrical signal and athird electrical signal, and also processing the third electrical signalto obtain a measured value of an optical power of said third opticalsignal (S3).

Where the optical signals are analog, the measurement step may derivethe time-averaged optical power of the signal.

Where the optical signals comprise bursts of digital data alternatingwith lulls, the measuring step may derive the average of the opticalpower of said optical signal averaged over the duration of theindividual bursts. More particularly, where the instrument is to be usedfor measuring power of optical signals comprised of “bursty” datastreams (such as ATM data signals), the measuring step may extract thepower only from the data bursts and not from any intervening series ofdigital zeros (i.e., lack of signal).

According to still another aspect of the invention, there is provided aportable test instrument for measuring an optical power of each of afirst optical signal (S1) and a second optical signal propagatingconcurrently in opposite directions in an optical transmission path in apassive optical network (16, 16/1, . . . , 16/9) between an optical lineterminal (10) and an optical network terminal (14/1, . . . , 14/9), theoptical network terminal (14/1, . . . , 14/9) being operative totransmit the first optical signal (S1) only if it continues to receivethe second optical signal (S2) from the optical line terminal (10), theportable test instrument comprising first and second connector means(22, 24) for temporarily connecting the portable test instrument intothe optical transmission path in series therewith, and propagating andmeasuring means (32, 38, 46) connected between the first and secondconnector means for propagating at least said second optical signal (S2)towards said optical network terminal (14), the propagating andmeasuring means (32, 38, 46) being operable to measure an optical powerof each of the first optical signal (S1) and the second optical signal(S2). The propagating and measuring means comprises coupler means (32)having first and second ports (28,30) connected to the first and secondconnector means, respectively, so that a path therebetween within thecoupler means completes the optical transmission path, a third port (36)for outputting a portion of each optical signal received via the secondport (30) and a fourth port (34) for outputting a portion of eachoptical signal received via the first port (28) and detection means (38,42, 44) coupled to the third and fourth ports for converting the opticalsignal portions into corresponding electrical signals.

Various objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription which describes embodiments by way of example only withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block schematic diagram of a portion of a passiveoptical network;

FIG. 2 is a simplified block schematic diagram of a power meteraccording to a first embodiment inserted into a branch of the network;

FIG. 3 is a detail view illustrating a modification;

FIG. 4 is a simplified block schematic diagram of a power meteraccording to a second embodiment;

FIG. 5 is a simplified block schematic diagram of a power meteraccording to a third embodiment; and

FIG. 6 is a simplified block schematic diagram of a power meteraccording to a fourth embodiment.

DETAILED DESCRIPTION

A portion of a passive optical network shown in FIG. 1 comprises a firstelement in the form of a central office optical line terminal (OLT) 10coupled by a 1:9 splitter 12 to a plurality of other elements in theform of optical network terminals (ONT) 14/1 to 14/9, each coupled to arespective one of the nine ports of the splitter 12 by one of acorresponding plurality of optical waveguides 16/1 to 16/9. (It shouldbe noted that, although nine terminals and a nine-port splitter areshown for convenience of illustration, there could be more or fewer inpractice.) The terminals use asynchronous transfer mode (ATM) or similarprotocol to encode the downstream (OLT to ONTs) and upstream (ONTs toOLT) digital data signals.

OLT 10 broadcasts to the ONTs 14/1 to 14/9 a downstream data signal (S2)at a wavelength of 1490-nm and a supplementary downstream signal (S3) ata wavelength of 1550-nm and, in known manner, encodes the 1490-nm datasignals (S2) for synchronization purposes, the encoding being decoded bythe ONTs and used to permit each of the ONTs 14/1 to 14/9 to sendupstream, to the OLT 10, 1310-nm digital optical data signals S1 in itsown unique time slot so as to avoid interference with signals from otherONTs connected to the same OLT 10. The signal S3, generally carryingcable television (CATV) information, is supplied by CATV source 11 shownconnected to the OLT 10 and combined with the data signals S2 in knownmanner. (In practice, if signal S3 is a CATV signal, it will be insertedlater).

If they do not receive the downstream signal (S2), and hence thesynchronization information, the ONTs cannot normally transmit. For afield technician to make measurements of either two, or all three, ofthe signals, therefore, it is necessary for the ONTs 14/1 to 14/9 tocontinue receiving the downstream signals from the OLT 10.

A test instrument 18 which allows the upstream and downstream opticalsignals to continue propagating, while measuring the power of theoptical signals S1, S2 and S3 at all three wavelengths, will now bedescribed with reference to FIG. 2, which shows the instrument 18connected into branch waveguide 16/9 between the splitter 12 and ONT14/9. The test instrument 18 comprises a casing 20 having first 22 andsecond 24 bulkhead connector receptacles or ports shown coupled to thesplitter 12 and ONT 14/9, respectively, connector receptacle 24 beingconnected to the ONT 14/9 by a short jumper 26.

Within the power meter casing 20, the connector receptacles 22 and 24are connected to first and second ports 28 and 30, respectively, of a2×2 optical coupler 32, having an approximately 80:20 splitting ratio,which ratio is approximately the same at all of the wavelengths to bemeasured (i.e., 1310 nm, 1490 nm, 1550 nm).

Thus, coupler 32 splits each of the signals S2, S3 and S1 received atports 28 and 30, respectively into two parts with a ratio of 80:20. The80 percent signal portions are each routed back to the other of the twoconnectors 22 and 24 while the 20 percent signal portions S1′ and S2′,S3′ are each routed to one of the corresponding third and fourth ports34 and 36, respectively, of the coupler 32.

Port 34, which receives the 20 percent portion S1′ of the signal S1 fromthe ONT 14/9, is connected by way of a filter 62, conveniently a 1310 nmbandpass filter, to a first photodetector 38 for detecting light atwavelengths nominally at 1310 nm. Port 36, which receives signalportions S2′, S3′ representing 20 percent of each of the 1490-nm and1550-nm optical signals from the OLT 10, is coupled to a 1×2 opticalsplitter 40, having an approximately 90:10 splitting ratio that isapproximately the same at all downstream wavelengths to be measured(i.e., 1490 nm, 1550 nm).

The 90 percent signal portions S2″ from splitter 40 are routed via thecorresponding output optical fiber from the optical splitter 40 to asecond bandpass filter 64, passing light within an approximately 15-nmwavelength band centered about 1490 nm and substantially attenuatinglight outside of this band (e.g. attenuation of greater than 40 dB at1550 nm). The output S2′″ of the second bandpass filter 64 is routed toa second photodetector 42, which detects light nominally at 1490 nm.

The 10 percent signal portion S3″ from splitter 40 is routed via thecorresponding output optical fiber to a third bandpass filter 66,passing light within an approximately 25-nm wavelength band centeredabout approximately 1550 nm and substantially attenuating light outsideof this band (e.g. greater than 20 dB for analog CATV signals, greaterthan 40 dB for digital CATV signals). The output S3′″ of the thirdbandpass filter 66 is coupled to the third photodetector 44, whichdetects light nominally at 1550 nm.

The three photodetectors 38, 42 and 44 supply their correspondingelectrical signals to an electronic measuring unit 46 which comprises aset of three similar amplifiers 48, 50 and 52 for amplifying theelectrical signals from photodetectors 38, 42 and 44, respectively.Power detectors 54 and 56 detect power of the amplified electricalsignals from amplifiers 48 and 50, respectively, and supply the powermeasurements to a processor unit 58 which, using an internalanalog-to-digital converter, converts them to corresponding digitalsignals which it processes to obtain the required optical powermeasurements, specifically power, and supplies the measurementinformation to a display unit 60 for display of the measurements in aconventional manner. The amplified signal from amplifier 52,corresponding to CATV signal S3, is supplied directly to the processorunit 58, i.e., without power detection, to provide a measure of averageoptical power.

The actual power measurements made by measuring means 18 will dependupon the nature of the signals being measured.

Where the optical signals are analog, the measuring means may bearranged to extract the time-averaged optical power of the signal.

Where the optical signals comprise bursts alternating with lulls, themeasuring means may be arranged to extract the optical power of thebursts.

If the optical signals comprise bursty digital signals, the measuringmeans may be arranged to the extract the optical power of the burstsaveraged over the duration of the burst. More particularly, where theinstrument is to be used for measuring power of optical signalscomprised of “bursty” data streams (such as the ATM data signals), themeasuring means may be arranged to extract the power only from the databursts and not from any intervening series of digital zeros (i.e. lackof signal). Such bursty data streams are typical of both the upstreamdata sent by an optical network terminal (ONT) to a plurality of opticalline terminals (OLTs) of a passive optical network (PON), and by the OLTto the plurality of ONTs.

Typically, the field technician will disconnect the link 16/9 to ONT14/9 at the home/premises etc. of the end-user at an existing“connectorized” coupling. The connector on the upstream part of the link16/9 will then be connected to a specified one (22) of the two bulkheadconnectors on the instrument, and the connector on the jumper 26 will beconnected to the other. Of course, if a connectorized coupling betweenparts of the link 16/9 is available, the jumper 26 may not be needed.

While the link is disconnected, emission of the upstream data signals atwavelength 1310 nm by the ONT 14/9 will normally cease, and will thenrecommence when the two connectors are connected to their respectivebulkhead connector receptacles 22,24 on the test instrument 18 and theONT begins to receive the 1490 nm signal again. Measurements can then betaken.

The fact that there will be a temporary disruption in the line as theinstrument 18 is inserted is not normally important, since the testinstrument will normally be used in service calls where a problem hasalready been indicated by the customer.

Once the test instrument is inserted into the line, between the splitter12 and the selected one of the ONTs 14/1 to 14/9 (see FIG. 1), 80%portions of the downstream data and video signals (i.e. at 1490 nm and1550 nm, respectively) will pass directly through to the ONT 14/9. TheONT, thus synchronized via the received data signal, will then be ableto emit its upstream (e.g. 1310-nm) data signal, an 80% portion of whichwill be sent upstream to the OLT 10, the other 20% portion beingdiverted to the detector 38.

It will be appreciated that the ratio of the coupler 32 need not be80:20. Embodiments of the invention may employ different ratios.Generally, lower ratios entail more attenuation while higher ratios aremore polarization-dependent. It should be noted, however, that preferredcouplers are available commercially that have a particular band ofwavelengths for which their ratios are substantially wavelength andpolarization independent.

It will be appreciated that the invention is not limited to themeasurement of optical power and to power meters, but could be appliedto the measurement of other parameters, such as optical spectrum,bandwidth utilization in the transmission path or link, and so on. Forexample, the coupler 32 could be combined with an optical spectrumanalyzer (OSA) which would replace the bandpass filters 62, 64, 66, thedetectors 38, 42 and 44, the measuring means 46, and the display 60, andthe optical splitter 40 would be replaced by a 2×1 coupler, preferablywith a 50-50 splitting ratio, to couple the ports 34 and 36 of the 2×2coupler 32 to the single input port of the OSA, thereby combining thetwo 20% signal portions. It will also be appreciated that the 2×1coupler inherently will introduce a loss, typically of 50% or more.

Of course, instead of the OSA, an alternative single-port device coupledto a 2×1 coupler could replace the components 38-66 of FIG. 2.

The bandpass filter 62 serves as a discrimination filter and isdesirable to avoid undesired effects caused by optical back reflectionof the 1550 nm signal, which can be acute when measurements are takenclose to the OLT 10. It may be omitted, however, if the apparatus, e.g.,test instrument, will normally be used close to the ONT terminal(s).

As illustrated in FIG. 3, which shows a modification to the instrument18, the splitter 40 and bandpass filters 64 and 66 may be replaced by awavelength demultiplexer 68 (e.g., a low optical crosstalk WDM coupler)which separates the signals according to their respective wavelengthsand supplies them to the detectors 42 and 44, respectively. It will benoted that FIG. 3 omits the bandpass filter 62, but it may be includedfor the reasons discussed above.

The electronic measuring unit 46 may be digital rather than analog, inwhich case it could be a suitably programmed microcomputer. Such digitalsignal processing potentially is more efficient, but also likely to bemore expensive.

It should be appreciated that, although each of the ONTs must receivethe optical signal S2 having a wavelength of 1490 nm, or it will nottransmit its own optical signal S1 of 1310 nm wavelength, it is notessential for the ONTs to receive the optical signal S3 at 1550 nmtransmitted by the OLT 10. Accordingly, FIG. 4 illustrates analternative optical test instrument 18′ which conveys only the 1490 nmsignal to the ONT 14/9.

The optical power meter 18′ shown in FIG. 4 comprises awavelength-division-multiplexer 68′ having a “combined” port connectedto receptacle 22 and two “divided” ports coupled to detectors 42 and 44,respectively. The WDM 68′ separates the 1490 nm and 1550 nm opticalsignals S2 and S3 according to wavelength and supplies them to detectors42 and 44, respectively. The 1550 nm detector 44 is shown connecteddirectly to the WDM 68′ (or via an optional bandpass filter 62, shown inbroken lines). The 1490 nm detector is shown connected by way of an80:20 coupler 40′ which receives the 1490 nm signal from the WDM 68′ andsplits it into first and second portions, namely an 80% portion and a20% portion, conveying the 80% portion by way of a secondcoupler/splitter 32′ to receptacle 24 for transmission to the ONT andconveying the 20% portion to the detector 42 via bandpass filter 64. Therespective outputs of the detectors 42 and 44 are coupled to theprocessor 58 for processing of their corresponding electrical signals.

Second coupler/splitter 32′ receives the 1310 nm signal S1 from ONT14/9, via the receptacle 24, and splits the 1310 nm signal into twoportions with a ratio of 80:20, conveying the 80% portion to the WDM 68′by way of the coupler 40′ and the 20% portion to a detector 38 (or viaan optional bandpass filter 66 shown in broken lines). The electricalsignal from detector 38 is processed by the processor 58′, as before.

Thus, with this arrangement, the 1490 nm signal from the OLT 10 passesto the ONT 14/9 via the fiber branch 16/9, the receptacle port 22, theWDM 68′, the two couplers 40′ and 32′, receptacle 24 and fiberjumper/branch 26. Providing it is receiving the 1490 nm signal, the ONT14/9 transmits its own 1310 nm signal, which follows substantially thesame return path to the OLT 10. The couplers 32′ and 40′ extractrespective small (20%) portions of the 1310 nm and 1490 nm signals fordetection and processing, as required, by detectors 38, 42 and processor58′.

It should be noted that, although the ONTs 14/1-14/9 need to receive the1490 nm signal or they will not transmit their 1310 nm signals, it isnot absolutely essential for the OLT 10 to receive those 1310 nmsignals. Consequently, it would be possible for the WDM 68′ to beadapted to allow the 1490 nm signals to pass, but block the 1310 nmsignals. Such an arrangement will now be described with reference toFIG. 5, which illustrates an alternative test instrument 18″ similar tothat shown in FIG. 4, in that it includes a WDM 68″ connected betweenreceptacle port 22 and detectors 42 and 44, detector 42 being connectedto WDM 68′ by way of a filter 64 and detector 44 being coupled,optionally, by way of a filter 62, shown in broken lines. Likewise,receptacle port 24 is coupled to detector 38 by way of a coupler 32″.

The test instrument 18″ differs from that shown in FIG. 4, however,because it does not have a coupler/splitter 40′ coupling the signal S2(1490 nm) to receptacle port 24, i.e., the signal S2 is not simplyconveyed through the power meter and a portion tapped off formeasurement. Instead, the test instrument 18″ usesoptical-electrical-optical (OEO) regeneration to regenerate the opticalsignal S2 from the digital equivalent of the received signal S2 suppliedto the processor 58″ and then transmits the regenerated optical signalto the ONT as the signal S2 it must receive in order to transmit its ownsignal S1.

Thus, test instrument 18″ has a modulatable optical source 70, such as alight-emitting diode (LED), driven by an electrical signal from theprocessor unit 58″ (produced by means of an internal digital-to-analogconverter) that is the optical equivalent of the electrical signalsupplied to the processor 58″ by detector 42. The optical output fromthe LED 70 is applied to coupler 32′ which passes it to receptacle port24 for transmission to the ONT 14/9. In this case, there is no opticalcontinuity through the power meter 18″, either upstream or downstream.Thus, the signals S2 and S3 are passed to detectors 42 and 44,respectively, via WDM 68″ and the signal S1 passes from coupler 32′ todetector 38.

It should be appreciated that the splitter/coupler 32′ in theinstruments of FIGS. 4 and 5 could be replaced by a WDM couplercorresponding to the wavelengths 1310 nm and 1490 nm.

FIG. 6 illustrates yet another test instrument 18′″ which is similar tothat shown in FIG. 3, in that it includes a WDM 68′, detectors 38, 42and 44 for the 1310 nm, 1490 nm and 1550 nm signals, respectively, 1490nm filter 64, a processor unit 58′″ and, optionally, filters 62 and 66.It differs, however, in that coupler 32 is omitted and replaced by acoupler 72 with a wavelength-selective reflective device 74, such as aBragg grating, written into the common path portion so as to reflect aportion, say about 5%, of the 1550 nm signal. The coupler 72 has ports72A and 72B connected to the instrument ports 22 and 24, respectively,and ports 72C and 72D connected to the WDM 68′ and detector 42,respectively, the latter by way of 1490 nm filter 64.

In operation, the coupler 72 receives the 1490 nm and 1550 nm signalsfrom receptacle 22 and conveys a portion, about 95%, of the 1550 nmsignal to port 24 for transmission to the ONT 14/9 and a portion, about5%, to the detector 42. The Bragg grating 74 reflects a portion of the1550 nm signal, about 5%, which leaves the coupler 72 via port 72C andis applied to WDM 68′. The 1310 nm signal from ONT 14/9 passes fromreceptacle port 24 to the coupler 72 and, leaving the coupler via port72C, is conveyed to the WDM 68, along with the 5% portion S3ρ of the1550 nm signal. The WDM 68′ separates the 1550 nm signal and the 1310 nmsignal according to wavelength and conveys them to detectors 38 and 44,respectively, optionally by way of bandpass filters 62 and 66,respectively.

Thus, the test instrument 18′″ provides an optical path therethrough forall three of the optical signals S1, S2 and S3.

It should be appreciated that the power meter 18′″ could be reconfiguredto reflect a portion of the 1490 nm signal rather than the 1550 nmsignal, i.e, by employing a different Bragg grating 74 and transposingthe detectors 42 and 44, with their filters, as appropriate.

It should also be appreciated that, in all of the above-describedembodiments, the filters could be bandpass filters or a combination oflow-pass and high-pass filters, and that the filters for the 1310 nm and1550 nm signals are optional.

Moreover, it should be noted that, in the embodiments of FIGS. 4, 5 and6, the programming of processor 58′, 58″ or 58′″ will be suitablymodified as compared with that for the embodiment of FIG. 2. Suchmodification will not be described herein since it should be apparent tothose skilled in this art.

It should be noted that, although FIGS. 1 and 2 show the third signal S3inserted into the OLT 10, it could be inserted later in the transmissionpath, for example between the OLT 10 and the splitter 12. This isespecially the case if the signal S3 is a CATV signal.

It will be appreciated that, although the above-described embodimentsare described as monitoring data signals S1 and S2 and CATV signals S3,the invention comprehends instruments and methods for monitoring otheroptical signals.

INDUSTRIAL APPLICABILITY

Portable apparatus embodying the present invention may be inexpensiveand easy-to-use. Ease of use is especially critical when they are usedfor testing FTTX networks since the maintenance field technicians aregenerally the same personnel who maintain wire telephone connections andrarely have had significant training in fiber-optic technology.

Although embodiments of the invention have been described andillustrated in detail, it is to be clearly understood that the same areby way of illustration and example only and not to be taken by way oflimitation, the scope of the present invention being limited only by theappended claims.

What is claimed is:
 1. A portable test instrument for measuring anoptical power of each of a first optical signal (S1) and a secondoptical signal propagating concurrently in opposite directions in anoptical transmission path (16, 16/1, . . . , 16/9) between two networkelements (10, 14/1, . . . , 14/9), at least one (14/1, . . . , 14/9) ofthe network elements being operative to transmit the first opticalsignal (S1) only if it continues to receive the second optical signal(S2) from the other (10) of said network elements, the portable testinstrument comprising first and second connector means (22, 24) fortemporarily connecting the portable test instrument into the opticaltransmission path in series therewith, and propagating and measuringmeans (32, 38, 46) connected between the first and second connectormeans for propagating at least said second optical signal (S2) towardssaid at least one (14) of the network elements, the propagating andmeasuring means (32, 38, 46) being operable to measure an optical powerof each of the first optical signal (S1) and the second optical signal(S2) wherein the propagating and measuring means comprises coupler means(32) having first and second ports (28,30) connected to the first andsecond connector means, respectively, so that a path therebetween withinthe coupler means completes the optical transmission path, a third port(36) for outputting a portion of each optical signal received via thesecond port (30) and a fourth port (34) for outputting a portion of eachoptical signal received via the first port (28) and detection means (38,42, 44) coupled to the third and fourth ports for converting the opticalsignal portions into corresponding electrical signals.
 2. A portabletest instrument according to claim 1, characterized in that where saidone of the network elements (14/1, . . . , 14/9) also receives via saidoptical transmission path a third optical signal (S3) at a differentwavelength from that of said second optical signal (S2), the propagatingand measuring means (46) further comprises means (40, 44, 52, 58; 44,58, 68) for measuring an optical power of the third optical signal (S3).3. A portable test instrument according to claim 1, characterized inthat where said one of the elements (14/1, . . . , 14/9) also normallyreceives via the optical transmission path a third optical signal (S3)at a different wavelength to that of said second optical signal (S2),the propagating and measuring means (46) further comprises a splitter(40) connected to the coupler means (32) for splitting correspondingoptical signal portions (S2′, S3′) into two parts (S2″, S3″), and filtermeans (64, 66) coupled to the splitter (40) for separating the two partsaccording to wavelength before supplying same to said detection means(38, 42, 44).
 4. A portable test instrument according to claim 1,characterized in that where said one of the elements (14/1, . . . ,14/9) also normally receives via the optical transmission path a thirdoptical signal (S3) at a wavelength different from that of said secondoptical signal (S2), said propagating and measuring means comprises awavelength discriminator (68) connected to the coupler means (32) forseparating at least a portion (S2′, S3′) of the combined second andthird optical signals (S2, S3) according to wavelength to obtaincorresponding separate parts (S2″, S3″) and supplying same to saiddetection means (38, 42, 44).
 5. A portable test instrument according toclaim 1, characterized in that the measuring means comprises a separatedetector (38, 42, 44) for each of the measured optical signals.
 6. Aportable test instrument according to claim 1, characterized in thatwhere the measured optical signal is analog, the measuring means (46) isarranged to extract the time-averaged optical power of the signal.
 7. Aportable test instrument according to claim 1, characterized in that,where the measured one (S1) of the optical signals comprises bursts ofdigital data alternating with lulls, the measuring means (46) isarranged to extract the average of the optical power averaged over theduration of the individual bursts.
 8. A portable test instrumentaccording to claim 1, characterized in that the measuring means (46)comprises custom circuitry.
 9. A portable test instrument according toclaim 1, characterized in that the measuring means (46) comprises asuitably-programmed microcomputer.
 10. A portable test instrumentaccording to claim 1, characterized in that said measuring means furthercomprises display means (60) for displaying measured optical powervalues.
 11. A method of measuring an optical power of each of a firstoptical signal (S1) and a second optical signal propagating concurrentlyin opposite directions in an optical transmission path (16, 16/1, . . ., 16/9) between two network elements (10, 14/1, . . . , 14/9), at leastone (14/1, . . . , 14/9) of the network elements being operative totransmit the first optical signal (S1) only if it continues to receivethe second optical signal (S2), the method comprising the steps of:temporarily connecting first and second connector means (22, 24) of aportable test instrument into the optical transmission path in seriestherewith, and using the portable test instrument to propagate at leastsaid second optical signal (S2) towards said at least one (14) of thenetwork elements, and measure an optical power of each of the firstoptical signal (S1) and the second optical signal (S2) wherein theportable test instrument comprises coupler means (32) having first andsecond ports (28,30) connected to the first and second connector means,respectively, so that a path therebetween within the coupler meanscompletes the optical transmission path, a third port (36) foroutputting a portion of each optical signal received via the second port(30) and a fourth port (34) for outputting a portion of each opticalsignal received via the first port (28) and detection means (38, 42, 44)coupled to the third and fourth ports for converting the optical signalportions into corresponding electrical signals.
 12. A method accordingto claim 11, characterized in that where said one of the networkelements (14/1, . . . , 14/9) also receives via the optical transmissionpath a third optical signal (S3) at a wavelength different from that ofsaid second optical signal (S2), the propagating and measuring step alsomeasures an optical power of the third optical signal (S3).
 13. A methodaccording to claim 11, characterized in that where said one of thenetwork elements (14/1, . . . , 14/9) also receives via the opticaltransmission path a third optical signal (S3) co-propagating with thesaid second optical signal (S2) at a wavelength different from that ofthe said second optical signal (S2), the measuring step includes thesteps of splitting a portion of the co-propagating optical signals intotwo parts, each comprising portions of the second and third opticalsignals (S2, S3), separating each of the two parts according towavelength, converting said parts into said second electrical signal anda third electrical signal, respectively, and also processing the thirdelectrical signal to obtain a measured value of an optical power of thethird optical signal (S3).
 14. A method according to claim 11,characterized in that where said one of the network elements (14/1, . .. , 14/9) also receives via the optical transmission path a thirdoptical signal (S3) co-propagating with the said second optical signal(S2) at a wavelength different from that of the said second opticalsignal (S2), said propagating and measuring step employs a wavelengthdiscriminator (68) connected to the coupler means (32) for splitting aportion of the co-propagating optical signals into two parts eachcorresponding to a respective one of the second and third opticalsignals, converting the parts to said second electrical signal and athird electrical signal, and also processing the third electrical signalto obtain a measured value of an optical power of said third opticalsignal (S3).
 15. A method according to claim 11, characterized in thatthe propagating and measuring step uses a separate detector (38, 42, 44)for each of the measured optical signals.
 16. A method according toclaim 11, characterized in that where the measured optical signal isanalog, the step of measuring said optical power values derives thetime-averaged optical power of the signal.
 17. A method according toclaim 11, characterized in that, where the optical signal whose opticalpower is measured comprises bursts of digital data alternating withlulls, the measuring step derives the average of the optical power ofsaid optical signal averaged over the duration of the individual bursts.18. A method according to claim 11, characterized in that the step ofmeasuring said optical power is performed using custom circuitry.
 19. Amethod according to claim 11, characterized in that the step ofmeasuring said optical power is performed using a suitably-programmedmicrocomputer.
 20. A method according to claim 11, further characterizedby the step of displaying the measured optical power value.
 21. Aportable test instrument for measuring an optical power of each of afirst optical signal (S1) and a second optical signal propagatingconcurrently in opposite directions in an optical transmission path in apassive optical network (16, 16/1, . . . , 16/9) between an optical lineterminal (10) and an optical network terminal (14/1, . . . , 14/9), theoptical network terminal (14/1, . . . , 14/9) being operative totransmit the first optical signal (S1) only if it continues to receivethe second optical signal (S2) from the optical line terminal (10), theportable test instrument comprising first and second connector means(22, 24) for temporarily connecting the portable test instrument intothe optical transmission path in series therewith, and propagating andmeasuring means (32, 38, 46) connected between the first and secondconnector means for propagating at least said second optical signal (S2)towards said optical network terminal (14), the propagating andmeasuring means (32, 38, 46) being operable to measure an optical powerof each of the first optical signal (S1) and the second optical signal(S2) wherein the propagating and measuring means comprises coupler means(32) having first and second ports (28,30) connected to the first andsecond connector means, respectively, so that a path therebetween withinthe coupler means completes the optical transmission path, a third port(36) for outputting a portion of each optical signal received via thesecond port (30) and a fourth port (34) for outputting a portion of eachoptical signal received via the first port (28) and detection means (38,42, 44) coupled to the third and fourth ports for converting the opticalsignal portions into corresponding electrical signals.